Intensive investigations over about the last decade have made a number of technologies available for NO.sub.x reduction. As briefly noted above, however, no one technology has proved to be effective for achieving the dual goals of fuel economy and low emissions of pollutants. The art has not developed a technology singly, or by combining two or more available strategies, to meet projected regulatory demands for fuel economy with low NO.sub.x emissions.
SCR catalysts are available to limit NO.sub.x emissions from diesel engines, but even though some success has been achieved in this area, no one has yet solved the problem of significant down times. SCR is effective in a practical sense when the exhaust gas temperatures are sufficiently elevated, and effective temperatures are not achieved during startup or idling. Moreover, its effectiveness depends on the operator maintaining a supply of reagent and monitoring the activity of the catalyst. These are limitations that are viewed as unfavorable to regulators. A national or regional policy, so limited, could be rendered ineffective simply by a significant number of operators failing to take the time to stop for a refill of reagent or to service their systems. Thus, the problem of refilling with reagent goes deeper than simply the convenience of the driver or other operator.
In addition to these factors, R. J. Hulterman, in "A Selective Catalytic Reduction Of NO.sub.x from Diesel Engines Using Injection Of Urea" (Ph.D. thesis, September 1995) describes a number of technical challenges for implementing urea-SCR, including clogging of atomizers, decomposition problems and system dynamics. And, as he points out, the incentives to employ ammonia-SCR may be less due to the hazards associated with it. Both U.S. Pat. No. 5,431,893, to Hug, et al., and European Patent Specification 615,777 A1, further address problems with SCR. The costs associated with these problems and the fact that SCR is ineffective over a significant portion of the operating cycle of a mobile engine have played a role in the limited successes seen today for SCR.
Primary measures, as noted above, have also been found to provide less than a full solution to the problem of NO.sub.x reduction with fuel economy. At a fixed load, as EGR is employed, both brake specific fuel consumption (bsfc) and particulate emissions increase. In this regard, see Psaras, et al, "Achieving the 2004 Heavy-Duty Diesel Emissions Using Electronic EGR and a Cerium Based Fuel Borne Catalyst", SAE Technical Paper Series No. 970189, 1997.
The art is awaiting the development of a process and apparatus that would permit reducing NO.sub.x without suffering undue penalties in either fuel consumption or particulate emissions, and to do so simply, reliably, economically and safely.